gnu: guile-sly: Use HTTPS URL.

* gnu/packages/guile.scm (guile-sly)[source]: Use 'https'.  Failing to
do that, we get redirected to 'https', which fails because GnuTLS is
missing.

gnu/packages/guile.scm

@@ -812,7 +812,7 @@ (define-public guile-sly
     (version "0.1")
     (source (origin
               (method url-fetch)
-              (uri (string-append "http://files.dthompson.us/sly/sly-"
+              (uri (string-append "https://files.dthompson.us/sly/sly-"
                                   version ".tar.gz"))
               (sha256
                (base32